A hollow fiber membrane is a tubular filament comprising an inner diameter, an outer diameter, with a wall thickness, usually porous, between them. The inner diameter defines the hollow portion of the fiber and is used to carry fluid, either the feed fluid to be filtered, or the permeating fluid, if the fluid being filtered contacts the outer surface. The inner hollow portion is sometimes called the lumen.
Hollow fiber membranes are used for diverse applications, including gas separation, reverse osmosis, ultrafiltration and particle and bacteria removal with microporous membranes. In these applications, the membrane acts as a permeable barrier, allowing the passage of the carrier fluid and some dissolved or dispersed species, and retaining other selected species due to differences in specie size, permeation rates, or other physical or chemical attributes.
In practical applications, fiber is cut or otherwise made to a specific length and a number of fibers gathered into a bundle. A portion of one or both ends of the fiber bundle are encapsulated in a material which fills the interstitial volume between fibers and forms a tube sheet. This process is sometimes called potting the fibers and the material used to pot the fibers is called the potting material. The tube sheet acts as a seal in conjunction with a filtration device. If the encapsulation process closes and seals the fiber ends, one or both ends of the potted fiber bundle are cut across the diameter or otherwise opened. In some cases, the open fiber ends are closed and sealed before encapsulation to prevent the encapsulation material from entering the open ends. If only one end is to be opened to permit fluid flow, the other end is left closed or is sealed. The filtration device supports the potted fiber bundle and provides a volume for the fluid to be filtered and its concentrate, separate from the permeating fluid. In use, a fluid stream contacts one surface and separation occurs at the surface or in the depth of the fiber wall. If the fiber outer surface is contacted, the permeating fluid and species pass through the fiber wall and are collected in the lumen and directed to the opened end or ends of the fiber. If the fiber inner surface is contacted, the fluid stream to be filtered is fed into the open end or ends and the permeating fluid and species pass through the fiber wall and are collected from the outer surface.
A variety of materials are used to form the seal. Epoxy resins and urethanes are commonly used as components for the seal. Thermoplastic polymers are another important class. These are polymers that can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application for which the filtration device is being used become more severe, the materials that can be used to form the seal becomes limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken urethane or epoxy based seals. The high temperature stripping baths in the same industry consist of highly acidic and oxidative compounds, which will destroy seals made of common polymers. Seals made from perfluorinated thermoplastics have exceptional resistance to chemical and thermal degradation and would provide excellent sealing material.
Membranes made of perfluorinated thermoplastic polymers are very useful in filtration applications requiring high degree of chemical and thermal resistance. To fully benefit from the properties of the perfluorinated thermoplastic membranes, a filter element using such membranes must be made of materials having similarly resistant properties. For high temperature operation it is preferrable that the melting temperature of the potting material be as close as possible to the melting temperature of the hollow fiber membranes. This will maximize the operational temperature because the operational temperature will be limited to approximately the lower melting temperature of the filter components. Also, it is difficult to pot perfluorinated thermoplastic membranes with dissimilar potting materials and obtain good bonding between the potting material and the perfluorinated thermoplastic membranes. For filtration of ultrapure solutions, exceedingly low levels of extractable residual matter is required of the filtration device. Perfluorinated thermoplastics are commonly used in applications which require very low extractable matter and a filter made entirely of perfluorinated thermoplastic materials would have an advantage in such applications. For these reasons, it is desirable to have a method of potting perfluorinated thermoplastic membranes in a perfluorinated thermoplastic potting material.
Manufacturing a filter element from thermoplastic hollow fiber membranes using thermoplastic polymers as a sealing material is more difficult than with typical resinous materials such as reactive epoxies and urethanes. Epoxies and urethanes used for this application as chosen to have good flow properties so that they can easily flow around the fibers to be sealed. These materials comprise low molecular weight reactive components, having low viscosities, which react to form the final pot after being flowed or otherwise loaded into a portion of a vessel containing a fiber bundle. Thermoplastic polymers are polymers which can be flowed and molded when heated and recover their original solid properties when cooled. Thermoplastic polymers are high molecular weight materials and have high viscosity. They do not easily flow around the fibers of a fiber bundle, and are not prone to flow uniformly around or through a mass of fibers. Thermoplastic materials have to be heated to melt or soften them in order to flow. The hot thermoplastic material can have detrimental effects on the fibers being sealed. Perfluorinated thermoplastics are particularly difficult to use as potting materials because of their high melting temperature and high viscosity. Perfluorinated thermoplastic polymers have to be heated to above their melting point to be extruded or injected. Too long a contact of the perfluorinated thermoplastic potting material heated above its melting point with porous hollow fiber membranes of similar melting points will cause melting and collapse of the hollow fibers. If the perfluorinated thermoplastic potting material cools too quickly as it is being flowed into the fiber bundle, it will not completely fill the interstitial spaces between the fibers. It will instead tend to form occluded volumes from the cooled potting material not being able to easily flow. These will result in weaknesses and possible leaks. Practitioners have attempted to overcome these difficulties in a variety of complicated schemes.
U.S. Pat. Nos. 4,980,060 and 5,066,397 discloses methods of making a filter element comprising a plurality of porous hollow fiber membranes of a thermoplastic resin, fusion bonded at the periphery of the end portions to form a terminal block. In one embodiment, thermoplastic hollow fiber membranes containing fine particle inorganic filler are dipped in a mixture of gypsum and water to seal the end openings. The end portions are dipped in a solvent for the inorganic filler to wash away the filler only from the surface of the end portions. The extraction operation may be effected efficiently by carrying by dipping the end portions of the membranes in the solvent while subjecting the solvent to ultrasonic treatment. The sealed end portions are arranged in a bundle in a lengthwise direction and the end portions are heated to at least the softening temperature of the resin used to make the membranes. The peripheries of the end portions of mutually adjacent membranes should be kept in contact during the heating step, as by winding a non-adhesive tape around the end portions prior to the heat treatment. The tape is removed after the heat treatment. In another embodiment, a powdery thermoplastic resin is applied to the peripheries of the end portions of the membranes. The resin is applied by dipping the end portions into a liquid, and then either putting the end portions wetted with the liquid in the powdery resin or spraying or spreading the powdery resin to the wetted end portions. When a plurality of membranes is dipped into the liquid, the inner membranes are not often not readily wetted. In such a case, the liquid may be subjected to an ultrasonic treatment. The powdery resin can also be applied by first preparing a mixture of the same liquid and the powdery resin and then dipping the end portions in the mixture or spraying or spreading the mixture onto the peripheries of the end portions. The membranes are then arranged in a lengthwise direction to form a bundle and the sealed end portions are heated to a temperature at least as high as the softening point of the resin used to make the membranes as described in the previous embodiment so that the end portions are fusion-bonded through the thermoplastic resin medium resulting from the powdery thermoplastic resin. In both these embodiments, the fiber bundle is subjected to complete extraction of the filler after the heat treatment. In another embodiment, unfilled membranes are heated without powdery resin material adhered to the peripheries while feeding an inert gas from openings at the opposite end of the fibers. There are several difficulties inherent in these complex operations. It would require exhaustive extraction to reduce the inorganic filler content to a sufficiently low level to be suitable for use in ultrapure fluid filtration, as is required for applications in microelectronic device manufacturing. Similarly extraction of the gypsum fiber end seal would increase cost and process difficulty. The fusion of fiber peripheries by heat requires keeping the fibers in mutual contact during the process. For large bundles, fibers nearer the outer portion of the bundle will have more pressure applied to them, in order to apply sufficient pressure to the inner fibers of the bundle. Non-uniform pressure can cause differences in fiber properties in the final potted bundle. The liquid-powdery resin application will require a further step to remove the liquid carrier. The flow of inert gas during fusion requires a separate seal around the fibers and a gas pressure supply and control apparatus.
U.S. Pat. No. 5,015,585 describes a process for making a homopolymer hollow fiber module by thermal bonding techniques which first requires insertion of a metal rod into the lumen of each hollow fiber to maintain its shape and integrity during the bonding process. After bonding is complete, the rods inserted into the fibers are forced through the ends and removed. This disadvantage with this process is the difficulty of reliably placing fine rods into very small diameter lumens and removing the rods after the bonding process.
In U.S. Pat. No. 5,284,584, a method is described in which thermoplastic resin is extruded at elevated temperatures to form a molten band of potting material that is directed onto the surface of a hollow fiber membrane fabric. The hollow fiber membranes constitute the weft of the fabric, with the warp consisting filaments which hold the membrane fibers is a space-apart relationship. The fabric is wound on an axis parallel to the fiber direction while simultaneously extruding the thermoplastic resin onto each bundle end to pot each of the two bundle ends in the thermoplastic resin, serving to seal the bundle end into an adjacent monolithic tube sheet. This method requires the added complexity of weaving a fabric from weak membrane fibers, operating an extruder and fabric. winding systems, and requiring control systems to integrate both activities.
U.S. Pat. No. 5,556,591 discloses a process for potting fibers within a body, comprising forming a bundle of hollow fibers within a hollowed portion of the body, the fibers being aligned substantially longitudinally and uniformly spaced, dispensing along the longitudinal axis of the bundle a molten thermoplastic composition at a contact temperature below the melting point of the fibers, but sufficient to lower the viscosity of the thermoplastic material to flow around all the fibers, applying a pressure differential to the body to assist flow, the bundle being at a temperature low enough to cause solidification of the molten thermoplastic substantially on contact, and forming an impervious seal around every fiber in the bundle and the interior of the body. This process requires an additional pressure differential means applied to the body during the process. Furthermore, dispensing a molten thermoplastic that solidifies on contact would be difficult for large bundles as the first material to solidify would prevent uniform flow of subsequent material.
U.S. Pat. Nos. 5,228,992 and 5,445,771 disclose enhancing hollow fibers by radiation treatment in order to cross-link the fibers throughout their structure and convert them to heat stable structures. The fibers are configured into modules or bundles into a loop or hourglass configuration and potted by injection molding of an intermediate section of the hourglass configuration. The pot is then cut across the potted area to form at least two modules. A special radiation treatment adds process steps to filter manufacturing and is not suitable for perfluorinated thermoplastics, as it will cause degradation of these polymers.
U.S. Pat. Nos. 5,505,858 and 5,662,843 disclose a process for making a filter element of polyolefin hollow fibers by two potting steps. A prepared bundle of fibers of a first polyolefin is immersed in a melted second polyolefin at a temperature not higher than the melting temperature of the first polyolefins and removed before the second polyolefin loses its fluidity and allowed to set. The resulting filter element with the attached second polyolefin is inserted into a second mold having a bottomless cup of nylon or polyolefin attached to the inside, with heating means attached to the outside. The mold contains either melted polyolefin of a low molecular weight which may be the same as the second polyolefin or a melted mixture of the low molecular weight polyolefin and ordinary polyolefin having an average molecular weight larger than that of the low molecular polyolefin. The filter element is removed and allow to set as described above. This method requires two separate heating and molding steps, thereby effectively doubling process complexity.
European Patent Application 0803281 A1 describes a potting process in which hollow fibers with their ends open are inserted into a melt of thermoplastic resin at a temperature not higher than the melting temperature of the raw material for the hollow fiber separating membranes. The melt is then cooled and solidified to form a seal in a half-bonded state showing no compatibility with the macromolecular material used to make the hollow fiber. Then, the open terminal ends of the bundle in the sealing part are opened by cutting or thermally melting the leading ends of the sealed part. This method is not available to all fibers. As described in the disclosure, fibers cannot be potted if the fibers have too high flexure or too low strength at rupture. Also, potting with opened ends can cause the fiber lumens to be filled with excessive potting material. That is, the potting material in the lumen will match the height of the pot, and leave no room for a cut to open the ends. In another embodiment, a paste of potting material dispersed in ethyl alcohol is injected into the open terminals of an outer tube containing a fiber bundle, baking the outer tube thus prepared and allowing the baked outer tube to stand and then cool off in the oven. The disadvantage of this method is that it requires explosion proof operation, due to the flammable nature of the alcohol.
The complex methods developed in the prior art indicate the difficulty of producing hollow fiber filter elements with thermoplastic polymers as potting materials. It is therefore desirable to have a simplified process to produce perfluorinated thermoplastic membrane filter elements in a low cost and efficient manner.
This invention is a simplified method for manufacturing a filter element of perfluorinated thermoplastic hollow fiber membranes potted with a perfluorinated thermoplastic polymer. The method comprises placing a portion of hollow fiber membrane lengths with at least one closed, into a temporary recess made in a pool of molten thermoplastic polymer held in a container, holding the fiber lengths in a defined position, preferably vertical, maintaining the thermoplastic polymer in a molten state so that it flows into the temporary recess, around the fibers and between the fibers, completely filling the interstitial spaces between fibers with the thermoplastic polymer. A temporary recess is a recess that remains as a recess in the molten potting material for a time sufficient to position and fix the fiber bundle in place and then will be filled by the molten thermoplastic. The temporary nature of the recess can be controlled by the temperature at which the potting material is held, the temperature at which the potting material is held during fiber bundle placement, and the physical properties of the potting material. A temporary recess can also be recess in a solid thermoplastic which will fill when the thermoplastic is heated to a temperature sufficiently above its softening or melting temperature to flow, and held at that temperature for the time necessary to fill the recess. The end of the fiber can be closed by sealing, plugging, or in a preferred embodiment, by being formed in a loop. The loop acts as a closed end, and the two fiber ends are away from the potting material. The fibers encapsulated with molten potting material are removed from the container and cooled to solidify the thermoplastic polymer and form the solid thermoplastic pot. Optionally, the container with the potting material and the potting fiber bundle is cooled, and the potting material and potted fiber bundle removed from the container. Excess thermoplastic polymer potting material is removed and the ends in the pot are opened by cutting through the potting material perpendicularly or at an angle less than 90xc2x0 to the long axis.
The process can be accomplished by arranging hollow fiber membrane lengths with at least one end sealed in a hollow body such as a tube with both ends open, wherein the hollow body becomes a part of the final filter element. The hollow body has a top and a bottom. The bottom will be placed into the temporary recess in the molten pool of thermoplastic polymer as described above. The long axis of the fibers are arranged substantially parallel to the long axis of the shell. The fibers are held vertically in place in the hollow body by fastener means and support structure means so that they do not rise by buoyancy or sink into the molten pool of potting material during potting. The fibers and shell are arranged so that one sealed end of each fiber is substantially near the bottom of the hollow body and each end is substantially in the same plane. A portion of the bottom of the hollow body with the fiber lengths in place is placed in a temporary recess in a pool of molten thermoplastic polymer held in a container, holding the hollow body in a defined vertical position, maintaining the thermoplastic polymer in a molten state so that it flows into the temporary recess, through and around the fibers and vertically up the fibers, completely filling the interstitial spaces between the fibers and the portion of the hollow body with the thermoplastic polymer. The fibers encapsulated with molten potting material are removed from the container and cooled to solidify the thermoplastic polymer and form the solid thermoplastic pot. Excess thermoplastic polymer potting material is removed and the ends in the pot are opened by cutting through the potting material and the hollow body perpendicularly to the long axis.
Optionally, the fiber lengths can be folded in a tight U-shape and each U-end placed substantially near the bottom of the hollow body with each U-end substantially in the same plane. Optionally, a mat of fibers can be made by arranging individual fiber lengths in a row perpendicular to the fiber length, the fiber lengths substantially equal distances apart, and joining the adjacent fibers ends at each end of the fibers with a continuous means, such as a tape. The continuous means can also seal the ends or one or both ends can be sealed separately. The mat can then be rolled on an axis parallel to the fiber lengths, either on a core or without a core. The rolled-up mat is then placed in the hollow body as described above. In either of these optional methods, the potting then proceeds as described above.